1. Field of the Invention
The present invention relates to a data demodulator for reproducing digital data from receive signals which have undergone differential phase-shift-keying (DPSK), and more particularly to a data demodulator wholly consisting of digital circuits, well suitable for digital cellular telecommunication systems of an M-channel (M is a positive integer) multiplex time division multi-access (TDMA) formula.
2. Description of Related Art
In a digital cellular telecommunication system subject to fading, it is recommended to use .pi./four-shift quadrature phase-shift-keyed (.pi./4 QPSK) signals in transmission and reception between the radio base station and any mobile unit (for instance according to "RCR Standards on Digital Cellular Telecommunication Systems", RCR STD-27A, January 1992, Research & Development Center for Radio Systems). A data demodulator for DPSK signals should correct frequency drifts (or frequency offsets) of receive signals while achieving appropriate symbol synchronization, and thereby improve the error rate of the demodulated data. Furthermore, a data demodulator for use in a TDMA system should quickly achieve symbol synchronization of burst signals, correct their frequency drifts and, where continuous reception takes place, accomplish said symbol synchronization and frequency drift correction taking account of resistance to noise and other pertinent factors.
One of the data demodulators embodying an attempt to address these problems is proposed in the 41st IEEE Vehicular Technology Conference, pp. 652-656, May 1991 and the Technical Report of the Institute of Electronics, Information and Communication Engineers, Japan, RCS89-64, March 1990. This data demodulator using adaptive carrier tracking (ACT), developed for use in mobile units which have to be compact and consume little power, detects the phase of.pi./4 QPSK signals by comparing the phase of the .pi./4 QPSK signals in an intermediate frequency band with that of a reference signal, and supplies the detected phase directly in a digital value. The data demodulator demodulates this digitized detected phase with digital circuits permitting large scale integration (LSI), and provides four-value code decision data.
The aforementioned ACT type data demodulator controls the frequency and phase of said reference signal independently of each other with an ACT circuit, an automatic frequency control (AFC) circuit and a phase/frequency control circuit to correct the carrier phase rotation of the .pi./4 QPSK signals on a symbol-by-symbol basis. This data demodulator, because it controls the frequency drifts of the .pi./4 QPSK signals relative to the reference signal and the reference phase for phase detection independently of each other, is quick in responding to burst signals such as TDMA signals, but has no function to optimize symbol synchronism. This data demodulator, moreover, requires more or less complex circuits for controlling the phase and frequency, such as said ACT circuit, AFC circuit and phase/frequency control circuit.